Children often amuse themselves playing with a funny toy of a very simple design. A button with two threads passed through its holes will rotate to and fro with a high speed if the threads are regularly stretched and slacked.
This principle of action underlies the new design of the expander demonstrated at Seoul exhibition of inventions. The work is done not so much by springs, but by a disk that rotates to and fro. First, the disk is wound up and then not only force should be applied, but even some handiness is needed to unwind it and to prevent stopping. It is much more interesting to use this body-builder than just to extend monotonously the springs of an ordinary expander.

As distinct from a simple springed expander, this one is provided with a disk that is additionally dynamized.